This invention relates generally to glass compositions employing one or more rare earth ions, and more particularly, to absorption filters which selectively absorb at 532 nm, 1064 nm or both.
The eye's response to light, its photopic response, is most sensitive in the green region of the visible spectrum. This response is at its maximum at around 550 nm. The eye's sensitivity decreases moving towards the red or blue regions of the spectrum.
Eyewear has been developed and is worn to protect the eye from damage from laser light. For years, there have been problems with eyewear designed for lasers which emit light in the green region of the spectrum. Eyewear available to date have tended to suffer from numerous problems including, (i) suppression of photopic transmission, (ii) low optical density, (iii) color distortion resulting from the need for a broad acceptance angle, and (iv) problems with color balancing. Color balancing has proven to be difficult partially because of the eye's photopic response.
Various glasses have been employed for laser protective eyewear. These glasses have utilized a variety of different compositions.
Rare earth oxides have been incorporated into glass for the purpose of wavelength selective filters, or for narrow pass filters. In U.S. Pat. No. 4,106,857, an optical filter is disclosed which incorporates a rare earth as one of the glass constituents.
As the amount of rare earth oxide in the filter increases, there is an increase in the optical density, Consequently, with lower concentrations, the thickness of the optical element must increase for effective eye protection. It has been difficult to obtain higher concentrations, with the glass remaining stable with regard to devitrification.
However, the use of rare earth oxides in glass has been limited to no more than 25-30 mole percent. In U.S. Pat. No. 3,971,723, a glass incorporating Tb.sub.3 O.sub.2 as one of the constituents has been reported. These glasses do not absorb at any useful laser wavelength, particularly in the visible or in the infrared.
There have been difficulties incorporating rare earth ions in higher concentrations, particularly for glass systems made in commercial quantities. Those rare earth glass systems employed as filters have suffered the limitation in that they absorb at fixed wavelengths which are not necessarily the wavelength of interest. This is the result of their electron transitions, resulting from rearrangement of inner shell shielded 4f electrons, being fixed at specific energy levels. In glass hosts, minor shifts in position of only a few nanometers can be obtained. The sharpness of the transitions have been employed advantageously to prepare rare earth containing band pass filters.
Interference-type filters can be designed to absorb at selective wavelengths in well collimated light. A limitation is that they exhibit strong viewing angle dependence, making them unsuitable for many applications.
Absorption-type filters do not suffer from this limitation. Further, these filters can be designed to have excellent thermal, chemical and mechanical properties, as well as offering the possibility of prescription eyewear.
An immediate need exists for laser safety eyewear, such as an absorption-type filter, that is opaque at the lasing wavelength of Nd:YAG (1064 nm) and frequency doubled Nd:YAG (532 nm). Furthermore, these filters should have optical densities from 3.0 to 5.0 at practical thicknesses, and possess high luminous transmission with minimal color distortion.